Show that, in a group G,∘(ab)=∘(ba) for all a,b∈G . Further show that if a and b commute, and ∘(a)=n1,∘(b)=n2 and (n1,n2)=1, then ∘(ab)=∘(ba)=n1n2 .
Adv.
Let G be a group, H≤G and g∈G . Show that if ∘(g)=n and gm∈H, where n and m are coprime integers, then g∈H .
Let G be a group, H≤G and g∈G. Show that if ∘(g)=n1n2 and gm∈H, where n and m are coprime integers, then g∈H .
Let G be a group and g∈G with ∘(g)=n1n2 , where n1 and n2 are coprime positive integers. Then show that there are elements g1,g2∈G such that g=g1g2=g2g1 and ∘(g1)=n1,∘(g2)=n2 . Further show that g1 and g2 are uniquely determined by these conditions.
Let G be a group such that the intersection of all its subgroups which are different from {e} is a subgroup different from {e} . Prove that every element in G has finite order.
If a group G≠{e} has no nontrivial subgroups, show that G must be a finite group of prime order.
Give an example of a group G having a subgroup H and two elements a,b such that Ha=Hb but aH≠bH .
Let H≤G, and a∈G . Let aHa−1={aha−1|h∈H} . Show that aHa−1 is a subgroup of G.
Suppose that H≤G such that whenever Ha≠Hb then aH≠bH . Prove that gHg−1⊆H for all g∈G .
If H and K are subgroups of orders p and n , respectively, where p is a prime number, show that either H∩K={e} or H≤K .
If G is a group and K≤H≤G , then show that [G:K]=[G:H][H:K] (assume finite indices). Hence deduce that if [G:H]=p, where p is a prime, then either H=K or H=G .
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